If you copy something that way in a terminal, then open the coconut interpreter, you can still paste it. But I can't seem to get a cursor inside the terminal at all once coconut's interpreter has fired up

Hmmm... it does seem like Coconut must be doing something strange with prompt_toolkit. I'll try to compare to IPython and see if I can figure out what's up. In the meantime, you can pass --style none to disable prompt_toolkit entirely.